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In bulk Sr_{2}RuO_{4}, the strong sensitivity of the superconducting transition temperature T_{c} to nonmagnetic impurities provides robust evidence for a superconducting order parameter that changes sign around the Fermi surface. In superconducting epitaxial thin-film Sr_{2}RuO_{4}, the relationship between T_{c} and the residual resistivity ρ_{0}, which in bulk samples is taken to be a proxy for the low-temperature elastic scattering rate, is far less clear. Using high-energy electron irradiation to controllably introduce point disorder into bulk single-crystal and thin-film Sr_{2}RuO_{4}, we show that T_{c} is suppressed in both systems at nearly identical rates. This suggests that part of ρ_{0} in films comes from defects that do not contribute to superconducting pairbreaking and establishes a quantitative link between the superconductivity of bulk and thin-film samples.

We have observed Shubnikov-de Haas oscillations in FeSe. The Fermi surface deviates significantly from predictions of band-structure calculations and most likely consists of one electron and one hole thin cylinder. The carrier density is in the order of 0.01 carriers/Fe, an order of magnitude smaller than predicted. Effective Fermi energies as small as 3.6 meV are estimated. These findings call for elaborate theoretical investigations incorporating both electronic correlations and orbital ordering. [ABSTRACT FROM AUTHOR]

The magnetoresistance and magnetic torque of FeS are measured in magnetic fields B of up to 18 T down to a temperature of 0.03 K. The superconducting transition temperature is found to be Tc=4.1K, and the anisotropy ratio of the upper critical field Bc2 at Tc is estimated from the initial slopes to be Γ(Tc)=6.9. Bc2(0) is estimated to be 2.2 and 0.36 T for B∥ab and c, respectively. Quantum oscillations are observed in both the resistance and torque. Two frequencies F=0.15 and 0.20 kT are resolved and assigned to a quasi-two-dimensional Fermi surface cylinder. The carrier density and Sommerfeld coefficient associated with this cylinder are estimated to be 5.8×10-3 carriers/Fe and 0.48mJ/(K²mol), respectively. Other Fermi surface pockets still remain to be found. Band-structure calculations are performed and compared to the experimental results. [ABSTRACT FROM AUTHOR]

Magnetic torque measurements using a microcantilever have been performed to investigate the superconducting phase of Sr2 RuO4 down to 40 mK. For high-quality single crystals with the transition temperature (Tc) of 1.48-1.49 K, an abrupt jump of the torque signal is found near 1.5 T in field parallel to the conducting RuO2 planes below ~0.8K. The jump corresponds to the first order transition recently revealed by magnetocaloric and magnetization measurements [Yonezawa et al., Phys. Rev. Lett. 110, 077003 (2013); Kittaka et al., Phys. Rev. B 90, 220502(R) (2014)]. Furthermore, weak diamagnetic and irreversible signals are found to persist above the first order transition up to 1.85 T. The result indicates the presence of a subphase boundary separating low- and high-field phases in the superconducting phase. The high-field subphase disappears when the field is tilted from the conducting planes only by a few degrees. Quantum oscillation measurements are also reported to clarify the strong sample-quality dependence of the high-field subphase. [ABSTRACT FROM AUTHOR]

The resistivity ρ and Hall resistivity ρH are measured on FeSe at pressures up to P = 28.3 kbar in magnetic fields up to B = 14.5 T. The ρ(B) and ρH(B) curves are analyzed with multicarrier models to estimate the carrier density and mobility as a function of P and temperature (T ≤ 110 K). It is shown that the pressure-induced antiferromagnetic transition is accompanied by an abrupt reduction of the carrier density and scattering. This indicates that the electronic structure is reconstructed significantly by the antiferromagnetic order. [ABSTRACT FROM AUTHOR]

We report Shubnikov-de Haas (SdH) oscillation measurements on FeSe under high pressure up to P=16.1 kbar. We find a sudden change in SdH oscillations at the onset of the pressure-induced antiferromagnetism at P∼8 kbar. We argue that this change can be attributed to a reconstruction of the Fermi surface by the antiferromagnetic order. The negative dTc/dP observed in a range between P∼8 and 12 kbar may be explained by the reduction in the density of states due to the reconstruction. The ratio of the transition temperature to the effective Fermi energy remains high under high pressure: kBTc/EF∼0.1 even at P=16.1 kbar. [ABSTRACT FROM AUTHOR]

The crystal structure, superconducting properties, and electronic structure of the novel superconducting 122-type antimonide BaPt2Sb2 have been studied by measurements of powder x-ray diffraction patterns, electrical resistivity, ac magnetic susceptibility, specific heat, and by ab initio calculations. The material crystallizes in a new monoclinic variant of the CaBe2Ge2-type structure, in which Pt2Sb2 layers that consist of PtSb4 tetrahedra, and Sb2Pt2 layers that consist of SbPt4 tetrahedra, are stacked alternatively, with Ba atoms located between them. Measurements of electrical resistivity, ac magnetic susceptibility, and specific heat revealed that BaPt2Sb2 is a superconducting material with a critical temperature of 1.8 K. The electronic heat capacity coefficient and the Debye temperature were 8.6(2) mJ/mol K2 and 146(4) K, respectively. The upper critical field and the Ginzburg-Landau coherent length were determined to be 0.27 T and 35 nm, respectively. The calculations showed that the material has two three-dimensional Fermi surfaces (FSs) and two two-dimensional FSs, leading to anisotropic transport properties. The d states of the Pt atoms in the Pt2Sb2 layers are the main contributors to the density of states at the Fermi level. A comparison between experimental and calculated results indicates that BaPt2Sb2 is a superconducting material with moderate coupling. [ABSTRACT FROM AUTHOR]

We report measurements of ac magnetic susceptibility Xac and de Haas-van Alphen (dHvA) oscillations in KFe2As2 under high pressure up to 24.7 kbar. The pressure dependence of the superconducting transition temperature Tc changes from negative to positive across Pc ~ 18 kbar, as previously reported. The ratio of the upper critical field to Tc, i.e., Bc2/Tc, is enhanced above Pc, and the shape of the Xac versus field curves qualitatively changes across Pc, dHvA oscillations smoothly evolve across Pc, indicating no drastic change in the Fermi surface up to 24.7 kbar. Three dimensionality increases with pressure, while effective masses show decreasing trends. We suggest a crossover from a nodal to a full-gap s wave as a possible explanation. [ABSTRACT FROM AUTHOR]

Nonequilibrium steady state conditions induced by a dc current can alter the physical properties of strongly correlated electron systems. In this regard, it was recently shown that dc current can trigger novel electronic states, such as current-induced diamagnetism, which cannot be realized in equilibrium conditions. However, reversible control of diamagnetism has not been achieved yet. Here, we demonstrate reversible in situ control between a Mott insulating state and a diamagnetic semimetal-like state by a dc current in the Ti-substituted bilayer ruthenate Ca3(Ru1-xTix)2O7 (x=0.5%). By performing simultaneous magnetic and resistive measurements, we map out the temperature vs current-density phase diagram in the nonequilibrium steady state of this material. The present results open up the possibility of creating novel electronic states in a variety of strongly correlated electron systems under dc current. [ABSTRACT FROM AUTHOR]